Friction material

ABSTRACT

A friction material, such as those belonging to the NAO or LS classes. The friction material is substantially free from copper and includes non-spherical particles in the form of powders and/or fibres each constituted by a preferably ferrous metallic core and by an at least partial coating of core formed at least partially or totally by tin and/or tin compounds, such as intermetallic Fe—Sn compounds.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.14/708,975, filed May 11, 2015, which claims priority upon ItalianPatent Application No. TO2014A000375, filed May 12, 2014, the entirecontents of each application herein being incorporated by reference.

TECHNICAL FIELD

The present invention relates to a friction material to be used for themanufacture of friction layers for friction elements such as brakingelements, for example brake pads or brake shoes for vehicles, and/orclutch disks that are asbestos free and more general preferablybelonging to the friction materials classes known as NAO (“Asbestos-freeOrganic friction material”) and LS (“Low Steel friction material”).

In particular, the invention relates to a friction material which allowsto reduce the wearing of the friction layer arranged on the frictionelement (braking element or clutch disk), as well as especially thewearing of the element to be braked, for example, the disc or drum brakeof a vehicle, or of the element that in use cooperates with the frictionelement, defined in technical jargon as a “friction partner”, whilemaintaining good characteristics such as sensitivity with respect to themating surfaces of the friction partner, which allows to avoid scratcheson the mating surface, and adequate braking capacity (friction stabilityparticularly when hot).

TECHNICAL BACKGROUND

The friction materials of the above type include five classes ofcomponents: a fibrous material base, a binder and a filler, one or morelubricants or friction modifiers, one or more abrasives. To a greatextent asbestos was used in the past as the fibrous material, whichmaterial however presents considerable environmental problems and haswell known toxic effects on human health and for this reason has beenbanned by legislation for a long time. This material has therefore beenreplaced by other materials, both inorganic, such as rock wool,wollastonite and fiberglass and organic materials such as aramid fibersand carbon fibers and metallic such as copper, tin, iron, aluminium andsteel powders or fibers and other metals or metal alloys such as bronzeand brass. The binder is in general a thermosetting polymer, forexample, based on phenolic resins. Various materials are used as afiller such as barite (barium sulfate), calcium carbonate, talc,magnesium oxide, vermiculite; as abrasives, zirconium silicate,zirconium oxide, alumina, silicon carbide, mica; as friction modifiersmetal sulfides such as molybdenum disulfide, iron sulfides, copper, tin,graphite and/or coke. Other classes of materials are then added insmaller percentages such as, for example, rubber in powder or granuleform, “friction dust”, other organic materials.

EP0151185 teaches a material, and a process for its manufacture,constituted by an iron powder coated at least in part with tin, which isintended to produce magnetic cores for electrical apparatuses by meansof sintering.

U.S. Pat. No. 8,536,244 relates to a friction material, in particular toa mixture or blend for the production of friction layers, whichcomprises metallic powders and/or fibers, fillers, lubricants andorganic compounds, wherein are present in combination tin, tin alloys ortin compounds (typically oxides) in percentages comprised between 0.5%and 50%, and copper in percentages comprised between 0.001% and about5%. Furthermore, the surface area of the tin particles or powders mustbe comprised between 1 and 5 m²/g and the tin can be as a whole or inpart constituted by a surface coating applied over a rounded (sphericalshaped) metallic core made of iron. In practice, this document suggeststhe use in friction material compositions of rounded particles having aniron core and a shell of tin or its alloys in combination with a certaincontent of copper, possibly being present only in traces.

However, in order to preserve the environment and to avoid possibledamage to human health, increasingly more often various national andinternational regulations require the use of friction materials that arenot only free of asbestos and heavy metals but also with a reduced orzero copper content.

The friction material according to U.S. Pat. No. 8,536,244 isfurthermore very expensive to produce.

There is therefore a need in the art to provide a friction material thatis free of asbestos, heavy metals (such as cadmium or antimony) andcopper, having good performance, reduced production costs and that iscapable of reducing the wear of the friction elements manufactured withit and especially of the elements (“friction partners”) that cooperatewith friction elements, particularly when these are made of cast iron orsteel.

BRIEF DESCRIPTION

An object of the present invention is to provide a friction material, inparticular belonging to the classes of friction materials known as NAOand LS, which is capable of overcoming the drawbacks of the prior artdescribed above and that is not too expensive to produce.

The invention therefore relates to an asbestos-free friction material,in particular belonging to the classes of friction materials known asNAO and LS.

In particular, the friction material according to the invention isformed from a composition or mixture or blend comprising at least onefibrous base including inorganic and/or organic or metal fibers, atleast one filler and at least one organic binder, which is substantiallyfree of copper (which may be present at most as an impurity) and whichincludes non-spherical particles, both in the form of powders and fibersfor example, constituted by a metallic, preferably ferrous core, havingan asymmetrical, in particular non-spherical shape, and coated at leastpartially by a layer of tin or tin compounds.

The non-spherical particles constituted by an asymmetrically shapedmetallic core which is at least partially coated by a layer of tin ortin compounds according to one aspect of the invention have agranulometry comprised between 0.2 and 600 microns, preferably between0.2 and 250 microns and a surface area of less than 1 m²/g, inparticular comprised between 0.1 and 0.3 m²/g, for a granulometryfraction below 63 microns, oscillating in the near about of 0.2 m²/g.

In addition, the asymmetrically shaped metallic core of each particleand also, as a consequence, of the particles themselves, have asponge-like conformation; the core is preferably constituted by iron orsteel (iron-carbon alloy); the bulk density of the particles is in thenear about of 2 g/cm³ and is preferably equal to 2.34 g/cm³.

According to a further aspect of the invention, the non-sphericalparticles, both in the form of powders and fibers and constituted by anasymmetrically shaped metallic core which is at least partially coatedby a layer of tin or tin compounds, include within the layer of tinintermetallic compounds between the tin and the metal that constitutesthe core. In particular, the metal core is constituted by iron or steeland tin is present within the partial or total core coating in the formof intermetallic iron-tin compounds of the type Fe_(x)Sn_(y) (where1≦x≦5, 1≦y≦3), for example FeSn and FeSn₂.

The above-mentioned non-spherical particles having a metallic,preferably ferrous core, which is partially or totally coated by a layerof tin and/or tin compounds and preferably containing Fe—Snintermetallic compounds, are present in the composition or mixture orblend of the invention in a percentage by volume comprised between 0.5%and 50%, preferably comprised between 3% and 20%, and the Sn content ofthe particles is comprised between 20% and 30% by weight of the quantityof particles present.

The composition of the friction material used to form the frictionmaterial of the invention may comprise from 0.001% to 10% by volume ofsolid lubricants, such as, for example, tin sulfides, such as SnS andSnS₂.

It is also desirable to include graphite and/or coke in the composition.

The graphite can be any known graphite that is commonly used in frictionmaterials.

The graphite (and/or coke) is added in an appropriately selected amountwhich is preferably comprised between 2% and 15% by volume of the totalfriction material composition.

The composition of the friction material used to form the frictionmaterial of the invention may include FeMeSn ternary intermetalliccompounds (where Me is a metal different from Fe).

Preferably, the friction material of the invention, in addition to beingessentially free of copper, includes non-spherical particles in the formof powders and/or fibres each constituted by an asymmetrically shapedferrous (iron or steel) core and by at least a partial coating of thecore which is formed at least partially or totally from Fe—Snintermetallic compounds.

The invention lastly relates also to a friction element, in particular abrake pad or shoe, presenting a layer of friction material made from thefriction material described above.

The invention further relates to a braking system comprising an elementto be braked constituted by a disc brake or shoe constituted by castiron or steel and at least one braking element constituted by a brakepad or shoe which is designed to cooperate by means of friction with theelement to be braked, wherein the braking element presents a frictionlayer which is intended to cooperate with the element to be braked andwhich is made using the friction material described above.

The other components of the friction material composition of theinvention can be components used in friction materials already known inthe art.

In particular, the fibrous base can be part of any organic or inorganicfiber which is different than asbestos or else a metallic fiber that iscommonly used in friction materials. Exemplary embodiments includeinorganic fibers such as fibreglass, rock wool, wollastonite, sepioliteand attapulgite, and organic fibers such as carbon fibers, aramidfibers, polyimide fibers, polyamide fibers, phenolic fibers, celluloseand acrylic fibers or PAN (Poly-Acryl-Nitrile), metallic fibers such asfor example, steel fibers, stainless steel, zinc, aluminium fibers, etc.

The fibrous base can be used in the form of either short fibers orpowder.

Numerous materials known in the art can be used as organic or inorganicfillers. Illustrative examples include calcium carbonate precipitate,barium sulfate, magnesium oxide, calcium hydroxide, calcium fluoride,slaked lime, talc and mica.

These compounds may be used alone or in combinations of two or more ofthem. The amount of such fillers is preferably between 2% to 40% byvolume based on the total composition of the friction material.

The binder can be any known binder commonly used in friction materials.

Illustrative examples of suitable binders include phenolic resins,melamine resins, epoxy resins; various modified phenolic resins such asepoxy-modified phenolic resins, oil-modified phenolic resins,alkylbenzene-modified phenolic resins and acrylonitrile-butadiene rubber(NBR).

Any one or combinations of two or more of these compounds may beemployed. The binder is included in an amount preferably from 2% to 30%by volume based on the total composition of the friction material.

The friction material components according to the invention aregenerally as follows:

-   -   binders    -   fillers    -   lubricants/friction modifiers    -   abrasives    -   fibres (inorganic/organic/metallic)    -   metallic powders    -   non-spherical metallic particles coated with Sn and Sn        compounds, preferably Fe—Sn intermetallic compounds.

The friction material of the invention is commonly produced by evenlymixing the specific amounts described above of fibrous base, binder andfiller in a suitable mixer such as a Henschel or Loedige Eirich mixer.

The non-spherical ferrous metallic particles of a preferablyasymmetrical shape and preferably at least partially coated with a layerof Sn and/or Sn compounds, preferably Fe—Sn intermetallic compounds,used in the invention are commercially available for other purposes andare produced by reacting an Fe powder already having the desiredcharacteristics of granulometry and shape with liquid phase tin, or elsethey can be produced in accordance with the method described inEP0151185.

It should be noted that, as evidenced for example byhttp://thelibraryofmanufacturing.com/powder_processes.html, metallicpowders naturally present variable and asymmetric shapes which are wellknown to persons skilled in the art, varying from spherical or simplyrounded shape to shapes such as spongy, angular, bowed, cylindrical,acicular or cubic.

In the present invention, non-spherical asymmetrically shaped ferrousmetallic particles are preferred both for their reduced surface area andbecause this shape is more easily obtained, unlike the spherical shapeused in U.S. Pat. No. 8,536,244 which is expensive and difficult toobtain.

The brake pad pressing is carried out at a temperature comprised between60 and 200° C. at a pressure from 150 to 1800 Kg/cm² for a durationcomprised between 3 and 10 minutes or else preforming the mixture withina die and thereafter pressing at a temperature from 130 to 180° C. at apressure from 150 to 500 kg/cm² (14.7-49 MPa) for a duration of from 3to 10 minutes.

The resulting pressed article is typically post-cured by means of heattreatment from 150 to 400° C. for a duration of between 10 minutes to 10hours, it is then spray painted or powder-painted, kiln-dried andpossibly machined where necessary to produce the final product.

The friction material of the invention can be utilized in applicationssuch as disk pad, brake shoes and linings for automobiles, trucks,railroad cars and other various types of vehicles and industrialmachines or in clutch disks.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to the following practical implementation examples and withreference to FIGS. 1 to 4 of the appended drawings, which illustrate:

FIGS. 1 and 3 illustrate the results of a diffraction and SEM (ScanningRlectron Microscope) analysis of the metallic particles coated in tinand its compounds as used according to one version;

FIG. 2 shows an SEM image of particles used, which highlights theirspongy appearance; and

FIGS. 4 and 5 give the results in graphical form of a braking efficiencytest.

DETAILED DESCRIPTION

Examples and comparative examples are reported here by way ofillustration and are not intended to limit the invention.

EXAMPLES

Three formulations were prepared marked with the letters O, A and B,according to the following table.

TABLE 1 TYPE COMPONENTS 0 B A Organic fiber 2.3 3.6 3.6 Binder 16.2 19.319.3 Rubber 2.3 2.3 2.3 Graphite 6.5 7.6 7.6 Fluorine compounds 3.1 3.13.1 Baryta 4.4 Strong abrasives 8.4 6.7 6.7 Sn sulfides 7.6 Metallicsulfides 5 5 Magnesium oxide 5.1 5.1 5.1 Chromite 4.1 4.1 4.1 Coke 20.520.5 20.5 Mild abrasives 4.5 4.5 4.5 Vermiculite 3.2 3.2 3.2 Steel fiber10.9 10.9 10.9 SN powder 3.2 Metallic powder 0.9 0.9 0.9 Non-sphericalmetallic particles 3.2 coated with Sn Total 100 100 100

The components shown in Table 1, indicating percentage values by volumeof the total volume of the mixture/blend, were evenly mixed in a Loedigemixer and pressed in a die under a pressure of 20 tonnes for 3 minutesat a temperature of 160° C., thereby being cured by means of 10 minutesof heat treatment at 400° C., thus producing a friction materialaccording to the invention indicated under the letter “B”, and materialsaccording to the known art, indicted under the letter “O”, and acomparative, containing Sn in the form of powders only, indicated underthe letter “A”.

Brake pads produced as described were subjected to the following tests:

-   Efficiency tests comprising: running in brakings, brakings at    different fluid pressures, “cold” evaluation braking (<50° C.) cold,    freeway simulation brakings, two series of high energy brakings    (FADE test) interspersed by a series of regenerative brakings.-   Wear test comprising various series of brakings with initial braking    temperatures (of the brake disk) comprised between 100 and 400° C.    and precisely:-   1000 brakings with an initial disk temperature of 100° C.-   1000 brakings with an initial disk temperature of 150° C.-   1000 brakings with an initial disk temperature of 200° C.-   1000 brakings with an initial disk temperature of 250° C.-   1000 brakings with an initial disk temperature of 300° C.-   500 brakings with an initial disk temperature of 350° C.

The test results are shown in FIGS. 4 and 5 of the attached drawings andin the following tables. FIG. 4 refers to the comparisonmixture/formulation “A” containing free tin within the mixture, whileFIG. 5 refers to the mixture/formulation of the invention, containingferrous particles covered with tin compounds.

TABLE 2 Mix “O” - State of the art Friction layer wear - pad [mm] BrakeInboard Pad Outboard Pad Average pads Temper- Wear per 1000 Wear per1000 wear per 1000 ature (C.) Stop (mm) Stop (mm) Stop (mm) 100 0.330.25 0.29 200 0.42 0.36 0.39 250 0.26 0.15 0.21 300 0.17 0.14 0.15 3500.21 0.18 0.19 Disk Wear [mm]: 0.136 Disk Wear [g]: 41

TABLE 3 Mix “B” - Invention Friction layer wear - pad [mm] Brake InboardPad Outboard Pad Average pads Temper- Wear per 1000 Wear per 1000 wearper 1000 ature (C.) Stop (mm) Stop (mm) Stop (mm) 100 0.27 0.23 0.25 2000.40 0.32 0.36 250 0.14 0.14 0.14 300 0.17 0.12 0.14 350 0.10 0.10 0.10Disk Wear [mm]: 0.078 Disk Wear [g]: 22.9

Comparing the disk wear for both test sets it can be seen that it islower in the formulation B (the one containing metallic particlescovered with Sn).

From the comparison between the formulation O and the formulation B, itcan be seen in particular that the disk wear is much improved (it issignificantly lower for formula B) from the formula B compared to theformula 0; also there is less pad wear.

From the graphs of FIGS. 4 and 5 it can instead be seen that the brakingefficiency of the formulation according to the invention is quitecomparable to formulations known in the art but containing tin.Comparison measurements were also made between the formulations A and Bwith regard to wear, confirming the results of the previous test betweenthe formulations O and B. In particular, the most evident results wereobtained regarding the level of disk wear, which is greatly reduced,according to the following comparison:

Disk wear formulation A - Comparison Start of Test g End of Test g8814.2 8807.3

Disk Wear formulation B - Invention Start of Test g End of Test g 8799.78794.9

As can be seen the disc wear was less than 30% in the case of theformulation of the invention.

Finally an investigation was made into the nature of the metallicparticles containing tin which when added to a formulation of the typesubstantially known in the art allow surprising results to be obtainedas revealed by the tests.

With reference to FIGS. 1, 2 and 3, it became clear that the materialused (ferrous particles obtained by mixing with tin, melting andcooling) presents itself in the form of powder particles or fibrousparticles having a ferrous core asymmetrically shaped and sponge-likeappearance (FIG. 2) which are at least partially coated with a layer ofFeSn intermetallic compounds which are clearly identifiable by means ofboth the diffraction analysis (FIG. 1) and the SEM (FIG. 3—the particleshave been cut). The surface area of the particles used, measured usingthe BET method, resulted surprisingly small (an average of 0.15 m²/g andequal to 0.2087 m²/g for those particles with a granulometry of lessthan 63 microns).

It is assumed that the beneficial experimental results obtained are dueto the fact that the tin present in the form of intermetallic compounds,which are weaker, or at least deposited upon ferrous particles with areduced surface area, “spreads” (during braking) over the frictionpartner (disc brake in the tests) better during use than with theformulation containing tin, thus obtaining the resulting significantreduction in disk wear. With respect to traditional tin-freeformulations the benefits are even more evident.

The objectives of the invention are then fully achieved.

1. An asbestos-free friction material comprising: at least one fibrousbase including inorganic and/or organic and/or metallic fibers, at leastone filler and at least one binder, the friction material beingsubstantially free from copper and including non-spherical particlesthat are constituted by a metallic core of asymmetric shape at leastpartially coated with a layer of at least one of tin or tin compoundsand having a granulometry comprised between 0.2 and 600 microns andwherein the non-spherical particles have a surface area between 0.1 and0.3 m²/g.
 2. The asbestos-free friction material of claim 1, furtherbelonging to classes known as NAO or LS.
 3. The asbestos-free frictionmaterial of claim 1, wherein the metallic core comprises iron.
 4. Theasbestos-free friction material of claim 1, wherein the metallic corecomprises steel.
 5. The asbestos-free friction material of claim 1,wherein the metallic core comprises one of iron or steel, and whereintin is present in the at least partial coating in a form ofintermetallic iron-tin compounds of a type Fe_(x)Sn_(y) (where 1<x<5,1<y<3).
 6. The asbestos-free friction material of claim 1, wherein themetallic core of the non-spherical particles are a ferrous metalliccores that are at least partially coated by the layer of at least one oftin or tin compounds and are present in a percentage by volume comprisedbetween 0.5% and 50%, and wherein an Sn content of said particles iscomprised between 20% and 30% by weight of an amount of presentparticles.
 7. The asbestos-free friction material according to claim 6,wherein the layer of at least one of tin or tin compounds contains Fe—Snintermetallic compounds.
 8. The asbestos-free friction materialaccording to claim 7, wherein the Fe—Sn intermetallic compounds arepresent in a percentage by volume between 3% and 20%.
 9. Theasbestos-free friction material according to claim 6, wherein the atleast partial coating of the ferrous metallic cores forming thenon-spherical particles also includes FeMeSn ternary intermetalliccompounds, where Me is a different metal than Fe.
 10. The asbestos-freefriction material according to claim 1, further including:lubricants/friction modifiers; abrasives; and metallic powders.
 11. Afriction element comprising a friction layer of the asbestos-freefriction material in accordance with claim
 1. 12. A braking systemcomprising: a member to be braked; a brake disc or a brake drumcomprised of cast iron or steel; and at least one braking membercomprising a brake pad or brake shoe, the braking member configured tofrictionally engage the member to be braked, wherein the braking memberhas a friction layer intended to cooperate with the member to be braked,the friction layer comprised of an asbestos-free friction materialaccording to claim 1.